Alternator Technical advice

The first time you'll know your alternator is faulty is when the RED light on your dashboard flashes up to tell you the battery isn't charging or worse case you have a flat battery.

The first check you need to make is to find out if you have charge from your alternator going to your battery, this can be done by using a volt meter directly across the terminal of your battery. If the alternator is charging you should see around 13.5 to 14 volts at the battery with the engine running. If the battery voltage is dropping with the engine running then your alternator isn't sending voltage to your battery.

The next thing you need to do is to find out why so first check is going to be to look for broken wires and then bad connections, once you've decided that the wiring to the alternator is sound the second check is to find out if your battery is any good and that it can actually hold a charge from your alternator. Most decent car accessory dealers can check the condition of your battery. 

Once you have decided your battery is good then the only thing left is the alternator, obviously something has gone wrong internally. The three main components of the alternator that generally affect the charging capabilities are the rectifier, the regulator and the carbon brushes.

Obviously to check the following parts you'll need to remove the alternator from your vehicle.

First thing to check for would be worn or broken carbon brushes, a small amount of wear is normal but if you can only just see the brush sticking out from the holder then that probably your issue, tell tale signs of this would be intermittent charging where the brushes are only just able to make contact with the copper sliprings. Replacing the brushes should sort out your issues.

If the brushes aren't worn then you have an electronic issue, either the rectifier has a blown diode or the regulator has stopped working. If the diodes are blown you can actually see the burnt diodes by looking at the underside of the rectifier but unfortunately there won't be any visible signs of failure on the regulator.

As a general rule of thumb it's usually the regulator that fails so by replacing the regulator and usually brushbox as well it'll normally repair your alternator.

Unfortunately you can't check an alternator off your vehicle without specialist equipment which means the only other way to check it after repair is to put it back on your vehicle.